Mini-fountain cold atom clock

Thesis

The use of atomic transitions as frequency references has lead to frequency becoming the most accurately-measurable quantity in modern science. This has allowed for the maintenance and dissemination of standard global timescales that continuously maintain consistency with each other to nanosecond precision, as well as advances in fundamental physics.

Current atomic fountain clocks reach fractional frequency instabilities of a few 10−16 after several of days of averaging. This is limited by the instability of several systematic effects that have been the subject of much scrutiny in order to further push the limits of current caesium, microwave-based technology. As this technology matures, it is natural to seek applications beyond National Metrology Institutes. However, caesium fountains are large, complex, and expensive pieces of equipment, that require a skilled team of people to build, operate, and maintain. The aim of this work is to design, build, and test a mini-fountain cold atom clock that achieves performance close to that of Primary Frequency Standards while reducing the size and complexity of the fountain physics package and optical control system.

This was achieved in several ways. A method of performing normalised clock state detection using only the MOT beams in the MOT chamber allows for the elimination of the large, traditional separated detection regions. Similarly, performing state selection in the MOT chamber with a waveguide, rather than a separate dedicated microwave cavity, allowed the centre of the MOT chamber to be only 14 cm below the centre of the Ramsey cavity. Cutting down on this space required careful simulation of the magnetic field, and a set of shields was designed to allow for a homogenous magnetic field throughout the interrogation region. A new Ramsey cavity design was integrated into the vacuum vessel itself, rather than being housed inside a separate, larger vacuum vessel. Finally, an all-in-fibre optical system for producing the cooling light was designed and implemented, which reduces the amount of free-space optical alignment to improve the robustness of the system.

The resulting rubidium fountain clock has a height reduced from 2.1 m to 0.8 m, and a footprint reduced from a 60 cm × 60 cm square to a 24 cm diameter cylinder, relative to NPL’s Primary Frequency Standards, achieved a short term fractional frequency instability of 9×10−13/ p τ/s, and the long-term instability is likely below 10−15, although an evaluation of the systematic effects is not yet complete.

Authors: Walby, S

• DOI: 10.5287/ora-xm6vekwkd
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: cold atom; atomic fountain; microwave clock
• Subjects: Atomic clocks